The present invention relates to a laser system and method of forming production volumes of economical and reliable blind vias in circuit boards, polymer based multichip modules and chipscale packages at speeds estimated to exceed 2,000 per second.
Laser drilled blind vias are constructed by passing and pulsing laser beam radiation over a pre etched window to remove dielectric material. The use of pre etched windows as a mask for laser drilling multilayer circuit boards is disclosed in U.S. Pat. No. 4,642,160. The method for making an interconnection down to the third level is also disclosed in U.S. Pat. No. 6,211,485 and copending U.S. Ser. No. 09/823,217, incorporated by reference. The novelty of the invention disclosed herein is that it does not follow the typical known methods for laser drilling circuit boards. Conventional thinking suggests that increased speed for laser drilling is enhanced only by increasing the pulsing rate. Furthermore, the conventional view is that increased pulse rates offers more peak power which is believed to be the primary element needed to remove materials including the dielectric materials used in the fabrication of circuit boards. While there is proof that the above described conditions are true for many dielectric material like the traditional FR4 materials used in most circuit board applications, the method is slow when compared to what is disclosed in this invention.
The method described herein takes into account the physics of materials, the physics of laser beam technology and ‘marries’ these physical conditions that blend to make the most open opportunity for what is termed a ‘wide process window’. This is best understood by noting the process and laser system about to disclosed can be operated by a person who has been trained to run a conventional circuit board mechanical drilling system instead of a higher level technician or engineer as is normal for most laser drilling systems currently in use in circuit board fabrication. The results of using a system as disclosed herein is significant cost saving where in some cases the output can be as high as 40 times laser drilling systems currently in use.
Laser drilling as a method for producing blind or buried microvias has risen to become the prevailing and most common method. As the microvia market and technology mature the demand will move toward making more z-axis interconnects down to level three as described in U.S. Pat. No. 6,211,485 and further down to levels four, five etc. Since there are other conditions that create difficulty in completing the fabrication of microvias especially as the feature sizes shrink down to below 0.076 mm (0.003″) a slot design is extremely important. These slot can also be multilevel (U.S. patent application Ser. No. 09/823,217) which also complicate the laser drilling and fabrication process. The most compelling reason for using slots as opposed to round opening in circuit board designs where the feature sizes are less than 0.076 mm (0.003″), is for ease in plating where the solutions can find a wider opening to flow in and out of the blind structure. There is, however, one other complication where a pulsing laser system is used to remove dielectric. That is, there is a constant and consistent overlap of the laser beam is normal accepted blind via laser drilling which can readily damage the blind via slot. The laser system for producing laser drilled blind microvias (especially slots) and the method described in this disclosure eliminate this issue and enable a high yield process, expected to be better than single-digit defects in parts per million.
Circuit board designs have become so dense that the only method for increasing component density is to use blind and buried microvias which are typically considered 0.127 mm (0.005″) or less in diameter. The cost for producing laser drilled blind vias has become cost-effective to the level that the laser is the method of choice. The speed of laser drilling blind microvias has also improved which is the primary reason that this method is the dominant technique. The next growth will be an extension of laser drilling blind microvias as the cost for producing these microvias is low enough to use as the method of choice for producing buried microvias. A buried microvia 35 is one that is internal in the circuit board not directly reaching the outer layers on either side but interconnecting two or more layers of internal circuitry as shown in FIG. 18. A buried microvia does not occupy surface area on the outside of an interconnecting substrate (circuit board, multichip module or chipscale package) and therefore allows very efficient interconnections to take place promoting improved component density on the surface.
The critical aspect of adopting either blind or buried microvia and especially variable depth interconnecting strategies is not the design but the cost to produce both of these advanced technologies. The costs must to be low enough that original equipment manufacturers (OEMs) will adopt blind and buried microvia technology plus the circuit board fabricators have to be able to effectively produce these advanced interconnections at a high yield and in volume. This invention, which includes a conveyorized laser drilling system (which also can be run in a manual or hand load mode), also provides a method which is extremely broad or open in process parameters which drops the operating costs to a level where it will support advanced consumer products such as 3rd generation (called 3G) personal devices such as mobile or cell phone and Internet connecting devices that are portable. At the other end of the spectrum are the very dense high layers large circuit boards that have reached and exceeded the possibility of mechanical drilling even microvia through the circuit board and now demand both blind and buried microvias. These are large circuit boards from Internet switch and router OEM companies known to be from 40 layers up to +50 layers.
In addition to the volume demand this invention pushes forward to provide for variable depth interconnections down to the fourth level, which allows for power and ground to be included for shielding high speed signal layer pairs, making the disclosed design and process very cost-effective.
The laser drilling system, method and interconnections described in this invention allow this segment of the circuit board fabrication process to be automated and follow all of the other segments which have been automated with conveyors moving panels in a constant mode for higher output. In order to accomplish the automation described in this invention, the following elements must be included which also increase yield and high output both at the same time which is considered non-normal or counter intuitive:                Conformal Mask (U.S. Pat. No. 4,643,160)        Variable Depth Interconnections to reflect off of buried copper surfaces (U.S. Pat. No. 6,211,485)        Single Pulse Beam Delivery (U.S. Pat. No. 6,211,485)        Constant “high speed” beam movement        Controlled laser beam energy (beam speed, pulse width, focus)        Compatible materials that do not move or slightly move during lamination and other process steps in fabrication.With the number of blind and buried microvias on a square meter totaling over 2 million and expected to double in the next three years, the method for manufacturing or fabrication these blind and buried microvias must both improve in output speed and yield. This invention serves both the demand for improved output and yield.        
The material of choice whose absorption matches the RF-Excited CO2 wavelength (10.6 micron preferred) is a non-woven aramid material that is called Thermount®. This material is produced by DuPont Fibers and is typically epoxy coated to match the characteristics of the most common circuit board material called FR4, which is normally included in a “hybrid” multilayer format since it is a cheaper and well accepted material. There is also a higher temperature version of this same kind of “hybrid” multilayer that uses polyimide instead of epoxy resin. Multiple material laminators or ‘treaters’ are processing version of the epoxy and polyimide coated Thermount®, including Arlon, Polyclad and Nelco (Dielektra) along with laminators in Japan and Taiwan. Arlon is coming out with a high performance version that uses another dielectric material which is a butyl rubber/cyanate ester resin rather than epoxy or polyimide.
Another material that was developed in conjunction with an earlier patent (U.S. Pat. No. 4,642,160) is currently commercially available through Isola Laminate Systems. This material called ‘resin coated copper or resin coated foil’ is a two part epoxy coated copper foil. The epoxy next to the foil is in the C stage or cured and the other epoxy coating is in the B stage or prepreg as noted by the circuit board industry. This material is falling out of favor for two reasons: first, it does have some crazing and cracking that can occur when processed, especially at thicker levels, and it does not allow for processing variable depth interconnection since it does not come in a pure prepreg or clad on either side. A fuller discussion will outline and detail how both prepreg and thin core laminates can be used to make interconnection in a cost-effective manner down to the fourth level and deeper.
A positive outgrowth of the invention describe in this disclosure is the ability to rapidly and cleanly remove large area of dielectric material. This is easily understood when the system is used to laser drill slots as shown in FIGS. 15 and 16. A natural outcome from this large area dielectric material removal is that an even larger area can be removed so that an integrated circuit or other semiconductor chip can be placed into the opening. Removing large area of dielectric material is quite time consuming with the point-to-point system that typically use a galvanometer and index the table many times. These systems use a nearly focused beam and therefore create more heat which will create charring or the flow of most resins that make up dielectric materials. These large areas as shown in FIG. 17 are used in “smart cards” which are made in huge volumes for consumers and demand a very low cost, which therefore much be rapidly processed.
The laser system and method described herein will rapidly and reliably produce drilling blind and buried vias at multiple depths, plus multiple depth slots described in this invention allowing blind vias to be economically introduced into interconnect packaging designs. In addition, it will lend itself to laser processing large openings for Chip-in-Board applications at greatly reduced costs due to the fast processing speeds.